The present invention relates to a start-up in-rush current protection circuit for DCDC converter, making use of capacitor and voltage clamping circuitry.
Typically, a DCDC converter is a device to control an external power MOS transistor to produce a certain regulated DC voltage. The external power MOS transistor is controlled by a signal from a driver block. The power MOS acts as a switch, turning on and off according to the driver signal which controls the gate of the power MOS transistor.
The driver block is usually turned-on by an Enable signal. During start-up, when power supply voltage rises, the driver block may have not been enabled. Due to parasitic capacitances of the power MOS transistor, it is possible to happen that the gate voltage of the power MOS transistor can not follow the fast rising of power supply voltage. This causes the output voltage to rise to a high voltage. This event may destroy the components connected to output voltage of DCDC converter.
Conventionally, as shown in FIG. 1, this problem is avoided by connecting a resistor Rp between the drain and gate of the power MOS transistor so that the gate voltage may follow the power supply voltage.
There are two drawbacks with the conventional circuit. The first drawback is that there is still possibility for the gate voltage to not follow the power supply voltage, especially when the resistance value of resistor Rp is large. The second drawback is that there is constant current consumed by the resistor after driver block is enabled. This current may be large if the resistance value of resistor Rp is small.